The measurement of the blood pressure of human beings is usually accomplished by applying a variable pressure through a pressure cuff to the brachial artery in the upper arm of the test subject. As the pressure is varied, the sound of the blood flow through the artery or the pressure pulses generated by the artery are usually measured and used as indications of various blood pressure values. These two techniques, known as the ausculatory and oscillometric methods, respectively, can both be automated.
With an automated ausculatory blood pressure monitor, a pump inflates the cuff such that the brachial artery, is occluded. Then the pressure is reduced in increments by the monitor and an audio transducer or microphone located on the arm of the subject adjacent the distal portion of the occluded artery converts the sound produced when blood flow is reestablished into electrical signals. These sound signals, known as "Korotkoff" or "K-sounds", are first detected when the cuff pressure is at a level known as the "systolic" pressure. The monitor continues to reduce the cuff pressure until these K-sounds disappear. The cuff pressure level where this occurs is known as the "diastolic" pressure. Once the systolic and diastolic pressures have been determined, the monitor deflates the cuff to zero pressure.
A typical automatic oscillometric blood pressure monitor also includes a pump which first inflates a pressure cuff around the upper arm of the subject to occlude the brachial cuff about the upper arm of the subject to occlude the brachial artery and then decreases the pressure in increments. However, instead of using an audio transducer on the arm, the oscillometric monitor uses a pressure transducer located in the monitor and connected to the cuff by a conduit. Even when the cuff has occluded the artery, the pumping of the heart of the subject causes the artery to flex or oscillate, resulting in pressure pulses that are received in the cuff and the transducer. As the pressure is decreased, the amplitude of the pulses increases to a maximum and then decreases again. The cuff pressure when the pulse amplitudes are at a maximum is known as "mean arterial pressure" (MAP). Also, the cuff pressures above and below MAP where the pulse amplitudes are predetermined fractions of the amplitude at MAP, represent the systolic and diastolic pressures, respectively.
The testing of automatic blood pressure monitors is typically very difficult because living subjects on which it may be tested may have different pressure levels at different times, and different subjects will have different pressure levels. Thus, the accuracy of the monitor with respect to its intended purpose can only be determined on a statistical basis by time consuming population studies. Since this is not possible on a production basis, typically only the electronics of the monitor are tested and a mercury manometer is used to test the accuracy with which the pressure transducer measures the cuff pressure. Then if the machine produces a correct reading for a single test subject known to have a stable blood pressure, it is assumed to be functioning properly. Obviously, the problem with this method of testing monitors is that there is no assurance that the monitor will operate to detect and measure pressure pulses in addition to the cuff pressure, or to measure blood pressures and pulse rates which are remote from those of the test subject. Also, a properly functioning monitor may be rejected if the test subject should have an unusual blood pressure level during the test.
It is known in the art to test automated ausculatory monitors by creating an artifical arm in the form of an inner cylinder filled with water in which sound waves approximating K-sounds are generated by a transducer. An outer layer of water in a flexible bag transmits the sound to the pressure cuff and microphone of the ausculatory monitor, and also simulates the compliance of a real arm. At present, however, there is no known method of simulating the arm of a test subject so that the pressure pulses of the oscillometric method can be generated for testing of a blood pressure monitor of that type.